1. Field of the Invention
This invention relates to methods for preparing a clean and stable silicon surface.
2. Description of the Related Art
Clean semiconductor surfaces are a key factor in preparing integrated circuits with high yields. There are two major types of contamination that occur on semiconductor surfaces: films and particulates. Particulates are materials that have readily defined boundaries, while films (for example, native oxide on a bare silicon surface) are layers of material on the surface of the wafer.
It is important to minimize or eliminate both films and particulates on the surface of the silicon wafer in order to optimize integrated circuit yields. Prior to epitaxial deposition and diffusion processes on bare silicon surfaces, particularly for processes conducted at less than about 850° C., it is important to have a clean silicon surface.
Particulates and films may be removed through cleaning. The standard cleaning method often involves one or more forms of an RCA cleaning procedure. The RCA Standard-Clean-1 (SC-1) procedure uses a mixture of hydrogen peroxide, ammonium hydroxide, and water heated to a temperature of about 70° C. The SC-1 procedure dissolves films and removes Group I and II metals. The Group I and II metals are removed through complexing with the reagents in the SC-1 solution.
The RCA Standard-Clean-2 (SC-2) procedure utilizes a mixture of hydrogen peroxide, hydrochloric acid, and water heated to a temperature of about 70° C. The SC-2 procedure removes the metals that are not removed by the SC-1 procedure. If an oxide-free surface is required, the silicon wafer is dipped into an aqueous solution of hydrofluoric acid to etch away the native oxide layer and, theoretically, obtain hydrogen termination. There are a large number of variations on RCA clean and hydrofluoric acid dips.
After cleaning, wafers are typically stored for a period of time before further processing. Silicon-fluorine and silicon-carbon bonds are often observed on the silicon surface after cleaning. The fluorine and carbon contamination on the surface may be detrimental to the thermal budget and/or the quality of the layer to be grown or deposited on the surface of the wafer.
If the silicon wafer is dipped in hydrofluoric acid as the last cleaning step (also known as an “HF last” step), the surface of the silicon is typically terminated mostly with a monolayer of hydrogen, largely Si—H bonds. The hydrogen-terminated surface prevents oxidation better than without any termination. However, the surface of a silicon wafer after an HF last treatment normally starts to reoxidize within about 20 minutes after the original oxide layer was removed, quickly forming a new 5 Å to 7 Å thick oxide layer on the surface of the silicon wafer. Even with the best cleaning processes currently known, a layer of native oxide forms within 48 hours, and, often the wafers cannot be further processed within that time. This will mandate a new HF dip or in situ vapor clean if an oxide-free surface is required for the next process step.
In an HF last, when the oxide layer is removed from the surface with a hydrofluoric acid solution as the final step in the cleaning procedure, the wafer surface has a tendency to have high levels of particles due to: 1) exposure to contaminants in the solution; 2) exposure to air at the air/liquid interface; 3) deposition of particles during the drying process; and 4) exposure to air during the time between the drying step and the time that the silicon wafer is placed in an inert environment.
U.S. Pat. No. 6,620,743 to Pagliaro et al. teaches a method for forming a stable, oxide-free silicon surface. The '743 patent teaches an optimized APM clean, followed by a dilute HF etch, then an in situ rinse, and a dry only spin dry. The method taught by the '743 patent achieve silicon surfaces with a desirable level of stability, but is disadvantageous in certain respects. For example, the method requires short time intervals between its processes, is time-consuming, and utilizes expensive equipment.
Accordingly, there is a need for improved methods of preparing a clean and stable silicon surface.